B2.2: Organelles And Compartmentalization Flashcards
What are organelles?
Discrete structures within a cell that are adapted to preform a specific function
Can be solid structures or combination of dissolved solutes
(Plasma membrane is considered a organelle)
What does no/single/double membrane mean?
What are some examples of each?
No membrane: not enclosed by a phospholipid layer-> solid structure in cyto/nucleoplasm
- ribosomes
- centrioles
- nucleolus
Single membrane: enclosed by a single phospholipid bilayer
- Vesicles+vacuoles
- RER/SER
- golgi
- lysosome
Double membrane: enclosed by 2 phospholipid bilayers
- nucleus
- mitochondria
- chloroplast
What structures in a cell do not meet the requirements of an organelle?
Cell wall:
Outside of cell membrane (extracellular)
Not in cell -> not organelle
Cytoplasm:
Cytosol -> liquid part of cytoplasm, 80% water and dissolved solutes
Many metabolic reactions happen in cytoplasm -> but is not specialized to preform a specific function -> not organelle
Cytoskeleton:
Composed of many filaments and tubules
Not discrete -> not organelle
Outline why post-transcriptional modification of RNA is not possible in prokaryotic cells
In eukaryotic:
mRNA must undergo modification before being translated ->
Parts are removed/altered before leaving the nucleus
In prokaryotic:
No nucleus -> no altering/modifications
Explain how eukaryotic cells evolved to have compartments
Through infolding and endosymbiosis -> become super efficient spaces where many activites can happen at once
Membrane is only permeable to small number of substances, barrier between aqueous solutions, a compartment with controlled conditions
What are the advantages of compartmentalization?
compartments are separate from environment of surrounding cytosol
Tailored to function of organelle
Enzyme and substrate can be localized -> higher concentration
Damaging substances separated
Optimal conditions maintained for certain processes (efficiency)
Large areas of membrane becomes dense with proteins for a specific process -> location and number of organelles can be altered depending on the requirements of the cell
What are the advantages of compartmentalization in the nucleus?
Allow for genes to be separated
- genes that need to be expressed in differentiated cells can be copied and released into the cytoplasm
- genes that don’t need to be expressed -> stay in nucleus/inactive
What are the advantages of compartmentalization in the lysosome?
High concentration of enzymes -> can digest/hydrolyse all types of biological polymers
All lysosomal enzymes -> acid hydrolyses -> active at low pH -> maintained in lysosome
Double protection:
compartment allows for protection of other organelles
Neutral pH of cytoplasm would prevent hydrolase from digesting everything
Explain phagocytosis/phagocytic vacuole
Phagocytosis:
Process by which a cell uses its plasma membrane to engulf a large particle giving rise to a phagocytic vacuole/phagosome
Type of endocytosis
Cell that does this -> phagocytes
Fuses with lysosome to breakdown pathogen
What are the advantages of the double membrane of the nucleus? (Envelope and pore) (what is pore)
(Look on notes (B2.2.6) for info on the nucleus)
The nuclear envelope:
Nucleus surrounded by 2 concentric membranes (inner (INM) and outer (ONM))
Both are phospholipid bilayers
ONM continuous with RER
Protect genetic material in the cell and separates it from substances/chemical reaction in the cytoplasm
Nuclear pores (complex):
Selective passageway where small polar molecules/ions/macromolecules can travel (x30 size of ribosome)
large pore as a result of double membrane
Very big because:
Protein needed for genome structure and function synthesized at ribosomes in cytoplasm -> nucleus
mRNA and tRNA formed at nucleus (transcription) -> exported to cytoplasm
(for translation)
Ribosomes in nucleolus -> into cytoplasm/ER
What are the advantages of the double membrane of the nucleus? (Cell division)
(Look on notes (B2.2.6) for info on the nucleus)
Nucleus disassembles and reformes each time (most) cells divide
Prophase -> chromosomes condense and nucleolus disappears -> nuclear membrane fragmented into vesicles -> nuclear pore complex disappears
Final steps of cell division -> vesicles formed by nuclear membrane surround the chromosomes -> vesicles around each chromosome fuse -> double membrane around chromosomes -> vesicles fuses with each other -> complete nucleus and pore complexes reassemble
What is the endomembrane system?
System of compartmentalized sacs within a eukaryotic cell that work together to modify processes and ship molecules within/out of the cell
All organelles composed of phospholipids
- phospholipids can move within bilayer:
-> membrane flexible
Which allows for:
Formation of vesicles
Materials to be taken in (endocytosis) or out (exocytosis)
Pinching in of the membrane during animal cell cytokinesis
Rough endoplasmic reticulum and ribosome structure (as a part of the endomembrane system)
RER:
Series of connected and flattened membranous sacs with bound ribosomes
Ribosomes synthesize polypeptides and release to the inside of RER
RER -> elsewhere in cell by vesicle -> usually Golgi apparatus
Ribosomes structure:
Made of dozens of proteins arranged on a scaffold of ribosomal RNA (rRNA)
rRNA facilitates the binding of mRNA and tRNA + catalyzes the formation of peptide bonds between amino acids
2 subunits
Once’s assembled -> bind to mRNA and synthesize protein
Ribosome structure (as a part of the endomembrane system)
Ribosome -> catalyze and synthesis of polypeptides during translation
Free: floating in cytoplasm and synthesizes polypeptides used in the cell (ex: lysosome)
Bound: attached to RER and synthesizes polypeptides that are secreted from cell or used in the cell membrane
Signal sequence -> dictate if free go to RER or not
Signal sequences occur during beginning of polypeptide -> signal recognition protein bind to polypeptide, pauses translation -> free ribosome bind to a receptor on ER, forms RER -> translation starts again, polypeptide chain moves inside the ER
Structure and function of Golgi apparatus
Structure:
Flattened membrane-enclosed sacs called cisternae
Polypeptides synthesized by bound ribosomes on RER -> vesicle -> Golgi
Vesicle fuse with Golgi at cis face (entry face towards the RER/nucleus, convex) -> receives protein/lipid filled vesicles
Polypeptides -> Golgi -> exit via vesicles budding from concave trans face (exit face, usually towards the plasma membrane)
Function:
Polypeptide modified into functional state
- add carbohydrates to make glycoprotein
- combine with other polypeptides to form quaternary structure of protein
After modification -> functional protein
Trans face of Golgi sorts, concentrates and packs protein into vesicles for transport to lysosomes, membrane, exterior, etc.
Formation, movement and fusion of vesicles
Membrane-bound sacs used for transport and storage
Formation:
Formed when the membrane bulges and pinches off (result of membrane flexibility/phospholipids being able to move within the bilayer)
-> Golgi, membrane (endocytosis), RER
Clathrin -> protein that creates a coat to help the phospholipids create a rounded shape while the vesicle is forming (very important!)
- coat formed -> receptor proteins on cell surface bind to target molecules -> target molecules are attached -> cytoskeleton protein help clathrin pit to deepen and seal off -> trap target molecule inside
Movement:
Vesicles move through the cell along cytoskeleton track (motor proteins)
Fusion:
Merging of vesicle with another organelle or with part of a cell membrane
Adds phospholipids to the target structure -> larger
Function of vesicles (the different types of vesicles)
Transport vesicles:
Transport proteins and lipids from one place to another within the cell
Secretory vesicles:
Transports protein and lipids from inside the cell to the plasma membrane
- integral protein within the plasma membrane (pumps; channels, adhesion/receptor proteins)
- hormones, neurotransmitters
Endocytic vesicles:
Formed by invagination of plasma membrane around an extra cellular substance during endocytosis
Deliver cargo to other organelles for further sorting/digestion (phagocytosis)
Other types include:
Peroxisomes - contain enzymes that digest fatty acids
Lysosomes - contain digestive enzymes which digest cellular waste/harmful substances
Explain cell fractionation
Progress in science follows development of techniques -> study of function of individual organelles possible because inventions like cell fractionation
Cell fractionation -> a process that involves a pure sample (contains only the specific organelle being studied) and breaking up a sample of tissues then centrifuging the mixture at different speeds
3 stages:
Homogenization -> cell sample broken up using
homogenizer (blender adjacent)
Filtration -> homogenate is filtered through a gauze
Ultracentrifugation -> filtrate is placed into a tube and tube placed in centrifuge (separates materials based on density)
- speed altered to separate different components of cell based molecular weight
Until this was discovered -> research limited
